Cs(TCNQ)(1.5) and [(CH3)(4)N](TCNQ)(1.5) were prepared with using chemicals and solvents purified carefully and were recrystallized under an inert atmosphere to minimize the effect of air. We investigated their magnetic properties and the effect of oxygen on the magnetism. It was found that both paramagnetic susceptibilities (chi(p)) of Cs(TCNQ)(1.5) and [(CH3)(4)N](TCNQ)(1.5) were very small, especially chi(p) of Cs(TCNQ)(1.5) was remarkably small such as 10 similar to40 x 10(-6) emu.mol(-1) all over the temperature region. After the specimen was exposed to air, chi(p) was measured again; for Cs(TCNQ)1.5, Xp increased clearly by about 20 x 10(-6) emu.mol(-1); however for [(CH3)4N](TCNQ)1.5, Xp was not changed. This result suggests that the magnetism of the Cs salt was sensitive to oxygen and that the Me4N salt is stable to air.